Treatment of metal surfaces



Aug. 13, 1968 L. E. WOLINSKI 3,397,132

TREATMENT OF METAL SURFACES Filed Oct. 16, 1964 METAL SURFACED FILMSTRUCTURE.

oncmuc VAPOR +0ARRIER GAS INVENTOR LEON EDWARD WOLIN SKI BY fizz/@444.

ATTORNEY United States Patent 3,397,132 TREATMENT OF METAL SURFAGES LeonEdward Wolinski, Butfalo, N.Y., assignor to E. I. du Pont de Nemours andCompany, Wilmington, Del, a corporation of Delaware Filed Get. 16, 1964,Ser. No. 404,234 10 Claims. (Cl. 204-165) ABSTRACT OF THE DISCLOSUREMetal surfaces are rendered more adherent to polymer coatings bysubjecting the metal surface to the action of a pulsating electricaldischarge at voltages of from about 1000 to about 100,000 and atfrequencies of from about 350 to 500,000 cycles per second, atatmospheric pressure in an atmosphere consisting of an inert gaseouscarrier medium which will sustain said electrical discharge and up toabout 5% by volume of an organic agent having a vapor pressure of atleast 0.25 mm. of mercury at 60 C., said agent being selected from thegroup consisting of polymerizable organic compounds, nonpolymerizableorganic compounds having replaceable hydrogen and perhalohydrocarbons.

This invention relates to the treatment of metal surfaces and moreparticularly to the treatment of metallic surfaces of flexible shapedstructures having at least one surface of metal, e.g., metal foils,metalliz/ed polymeric film structures, and metal foil-organic polymericfilm laminates, to produce improved structures.

A number of problems arise in the utilization of shaped structureshaving a metallic surface such as metal foils, rigid metal structures,as well as various plastic sheets or films bearing a metallic surface.In the fabrication and utilization of various metal coated plastic filmsin condensers, for example, difficulties arise through prematurebreakdown of the metal coated film by the action of corona or thereresults a corona breakdown at a lower voltage than is desired.Frequently it is desirable to adhere either a metal coated plastic filmor a metal foil to other surfaces or structures with very strong bondswhich will withstand exceedingly heavy usage, and available technologydoes not provide the extremely firm bonding that is required for suchapplications. For example, organic films deposited on conductivesubstrates by glow discharge polymerization show a tendency to peel orseparate from the substrate (Electrical Properties of Thin OrganicFilms, Bradley and Hammes, Radiation Research Corporation, Journal ofthe Electrochemical Society, volume 110, No. 1, January 1963). Further,such processes are not well adapted to large scale commercial operation.In still other situations, the bright metal surface on various metalstructures is subject to corrosion by various chemical actions orthrough the effects of weathering under adverse atmospheres. Still othermetal structures such as flat silverplate show the effect of tarnishingfrom the adverse atmospheres in which such articles may be stored.

It is therefore an object of this invention to provide a simple andeconomical process for rendering various shaped structures havingmetallic surfaces resistant to such adverse effects as noted above. Afurther object of this invention is to provide a simple and economicalprocess for treating the metal surface of flexible metal-surfacedstructures such as metal foils, metallized film, metal foilto-filmlaminates and the like to produce improved structures. It is a stillfurther object to provide various shaped structures having metallicsurfaces with greatly improved properties. The foregoing and relatedobjects will more clearly appear from the detailed description whichfollows.

These objects are realized by the present invention which, in essence,comprises subjecting the metal surface of a metal-surfaced shapedstructure to the action of an electrical discharge at substantiallyatmospheric pressure in a gaseous atmosphere containing up to about 5%by volume of the vapor of at least one organic agent having a vaporpressure of at least 0.25 mm. of mercury at 60 C. in an inert gaseouscarrier medium which will sustain said electrical discharge.

In the preferred embodiment of this invention, illustrateddiagrammatically in the accompanying drawing, a continuous web offlexible pellicular structure having a metal surface such as a plasticfilm bearing a thin metal coating, a laminate of a plastic film such aspolyester film to a thin metal foil such as aluminum, zinc, silver,chromium, copper, etc., or metal foil per se, is continuously passedbetween a set of spaced electrodes consisting of a rotating metal roll 1which is connected electrically to ground, and one or more stationaryhollow metal tubes 2 disposed parallel to the longitudinal axis of theroll and closely and uniformly spaced from the surface thereof. Thetubes are each connected electrically to a suitable power source whichsupplies to each tube electrode an alternating (or pulsating direct)current of from 0.3 to 5.5 r.m.s. (root mean square) amperes at avoltage in excess of about 2000 volts, and at a frequency within therange of from about 200,000 to about 500,000 cycles per second. Amixture of an inert carrier gas which will sustain an electricaldischarge, e.g., nitrogen, and the vapor of a polymerizable organicmonomer having a vapor pressure of at least 0.25 mm. of mercury at 60C., e.g., acrylonitrile or glycidyl methacrylate, said vaporconstituting not over 5% by volume of the mixture, is continuouslyfed-to the hollow interior of the tube electrodes through distributorducts 3 and issues from the tubes at the gap between each tube and thegrounded roll through suitable openings distributed along the length ofthe tubes whereby the electrical discharge across said gap and onto theexposed metal surface takes place in an atmosphere containing saidvapor. The mixture of nitrogen and vapor may, of course, be introducedinto the reaction zone through one or more tubes separate from theelectrode assembly and the stationary electrodes may be of solidconstruction. The assembly just described is suitably enclosed in achamber 4 maintained at atmospheric pressure and provided with thenecessary openings to facilitate maintenance of the vapor-containingatmosphere in the treating zone and controlled exhaust therefrom, and tominimize operational hazards.

In carrying out the surface treatment of this invention the potentialdifference between the electrodes providing the electrical discharge mayvary from very low voltages in the order of 1000 volts up to pulsatingvoltages of 100,000 and above. In general, however, it is preferred tomaintain the voltage in excess of about 2000 volts. Frequencies from 350cycles per second up to 500,- 000 cycles per second and above can beused. Frequencies in the range of 200,000 to 500,000 cycles arepreferred for many applications in order to obtain effective treatmentat commercially acceptable exposure times. For other applications,frequencies in the range of about 1000 cycles to 50,000 cycles providevery satisfactory results.

While the current to the electrodes may range up to 5.5 RF amperes ormore, for optimum results a range of about 0.3 RF amperes to 3.5 RFamperes is preferred. Power to the electrodes may range from about 1watt per lineal inch to the electrode length to 400 watts or more perlineal inch of the electrode length.

Any suitable means known to the art for providing an electrical stressfield (i.e., an electrostatic field) of alternating or pulsatingcharacter between spaced electrodes (e.g. a high frequency sparkgenerator of the type hereinafter identified or a motor generator setup)may be employed as the power source in the electrical dischargetreatment of this invention.

The electrodes are preferably spaced from about .01 inch' to about 0.125inch; Useful results can be obtained when the electrode gap is as low'as 0.005 inch to as much as 0.25 inch provided suitable adjustments insuch features as amount of current. electrode dimension and exposuretime are made. Time of exposure to the electrical discharge treatment isnot especially critical and effective treatments are realized atexposure times as short as 1 10- second and no adverse effects are notedat times as long as 60 seconds. Preferably, the exposure time should benot less than 4 l0 second. For economic reasons, exposure times as shortas possible consistent with effective treatment would normally beemployed.

Any organic agent selected from the group consisting of polymerizableorganic compounds, nonpolymerizable organic compounds having replaceablehydrogen and perhalohydrocarbons having a vapor pressure of at least0.25 millimeter of mercury at 60 C. may be employed for purposes of thisinvention. Typical examples of suitable polymerizable compounds includeacrylonitrile, glycidyl methacrylate, methyl methacrylate,cyclopentadiene, styrene, p-chlorostyrene, vinyl butyl ether, methylvinyl acetate, l-hexene, n-vinyl-Z-pyrrolidone, ethylene imine,tetrafluoroethylene, hexafluoropropene, dichlorodifluoroethylene andacrylic acid. Typical nonpolymerizable compounds having a replaceablehydrogen include xylene, hexane, cyclohexane, chloroform,tetrahydrofurane, diethylsulfone, tetramethylxylene,tetraisopropyltitanate, methylamine, ethylamine, dimethylamine,diethylene triamine and butylamine. Typical perhalohydrocarbons includecarbon tetrachloride, trichlorofluoromethane, dichlorodifluoromethane,monochlorotrifiuoromethane, tetrachlorodifluoroethane,trichlorotrifiuoroethane, dichlorotetrafluoroethane,bromotrichloromethane, dibromodifluoromethane,dibromomonochlorotrifiuoroethane, dibromotetrafluoroethane andmonochloroheptafluoropropane.

Carrier gases which are inert, that is, do not interfere with the actionof the organic vapor such as nitrogen or carbon dioxide, are preferablyemployed to facilitate transport of the organic vapor to the electrodegap or to facilitate discharge when gases having high dielectricstrength are used as the modifying agent. In some instances ammonia andvarious volatile amines may be employed as a carrier either along orwith nitrogen or carbon dioxide.

The process of this invention is particularly adapted to the treatmentof metal foils such as those based on aluminum, steel, copper, tin,etc., as well as various metallized plastic films such as films ofpolyethylene terephthalate bearing a coating of aluminum or zinc orlaminates of a plastic film to the metal foils. However, it is alsowithin the scope of the process of this invention to treat rigidstructures having a metal surface, using a suitable arrangement ofelectrodes as will be evident to persons skilled in the art.

The following specific examples will serve to further and more fullyillustrate the principles and practice of this invention.

Example 1 A one-mil thick film of biaxially oriented polyethyleneterephthalate, heat set at 200 C., and which bore a 0.0004 mil coatingof aluminum on one surface was drawn through an apparatus as illustratedin the accompanying drawing at a speed of feet per minute. Theelectrodes of the apparatus were connected to a high frequency sparkgenerator (Model HFSG-Lepel High Frequency Laboratories, Inc.). Thestationary electrodes were spaced 0.04 inch from the surface of the rolland the power setting of the generator was set at 70 corresponding tothe current of approximately 1.3 RF ainperes to the electrodes. Anatmosphere of about 3% by volume of trichlorofiuoromethane in nitrogenwas maintained between the electrodes.

The treated metallized film was made up into a capacitor and tested forresistance to corona breakdown. It was observed that the treated filmshowed no evidence of corona breakdown until a voltage of 1100 wasreached. The same metallized film without the electrical dischargetreatment showed a breakdown at 450 volts.

The capacitor for the evaluation of the treated film is constructed asfollows: Each capacitor is constructed of two strips of Z-inch widepolyethylene terephthalate film with the coating of the metalapproximately 0.004 mil thick on one side. The metal coating is 1%inches in width on each surface. The two strips of metallized film areinterwound with two strips of /4 mil aluminum foil 1% inches in width.The capacitor is then subjected to the corona starting voltage which isindicated by a cathode ray oscilloscope. The increase in startingvoltage permitted in the tests with a capacitor made from the test filmand a corresponding control made without electrical discharge treatingare compared.

Examples 2-5 Corona Discharge Metal Gaseous Example Surface Atmos-Treated Non-Treated phere Film Control (V olts) (Volts) 2 Zinc CF3G11,100 450 3 Copper C2FB 1,000 425 4- Silver. (3213501.... 1,000 475Chr0m1nm CCIQFL... 1,050 470 Example 6 A strip of aluminum foil threemils thick was drawn through the apparatus as in Example 1 at a rate of10 feet per minute. An atmosphere of nitrogen and acrylonitrile wasprovided in the space between the electrodes by bubbling a stream ofnitrogen through acrylonitrile (approximately 1% of acrylonitrile byvolume) maintained at room temperature at a rate of about four cubicfeet per minute. There was then applied to the treated. foil a thinlayer of epoxy resin (Epon 1004 with Epon U Curing Agent) and the resinbearing surfaces were pressed together to form a laminate. The laminatewas pulled apart on a Suter Tester and showed a bond strength of over3600 grams per inch. A control aluminum foil which had not been giventhe treatment in the electrical discharge showed a bond strength of 480grams per inch; a second control aluminum foil which had been drawnthrough the electrical discharge but in the absence of an atmosphere ofacrylonitrile showed a bond strength of about 600 grams per inch.

Example 7 Following the procedure of the preceding example, a strip oftwo mil thick cold rolled steel was drawn between the electrodes oftheapparatus described in Example 1 in an atmosphere of nitrogen andglycidyl meth acrylate; The resulting treated foil was made into alaminate through the use of Swift X-7071 epoxy resin adhesive and thelaminate was then pulled apart on a Suter Tester. The laminate showed abond strength of over 4000 grams per inch; a control foil which had notbeen subjected 'to the electrical discharge treatment showed a bondstrength of 500 grams per inch; another control foil which had beensubjected to an electrical discharge treatment but in the absence of thegaseous atmosphere of glycidyl methacrylate showed a bond strength ofabout 1500 grams per inch. Still another foil exposed for a comparabletime to an atmosphere of glycidyl methacrylate under reduced pressure ina glow discharge and made into a laminate as described above showed abond strength of 800 grams per inch.

Example 8 A two-mil strip of cold rolled steel bearing a chrome coatingof about 0.001 mil was drawn between the electrodes of the apparatus asin Example 1 at a rate of feet per minute in an atmosphere of nitrogenand diethylenetriamine. Power was supplied by a motor generatordelivering 2500' volts to the electrodes at a frequency of 3000 cycles.A corrosion test wherein a strip was dipped into a brine solution whenalternately permited to dry in air with the alterations taking placeover a 10-hour period. At the end of this time there was no indicationof any formation of rust spots indicating corrosion on the treatedstrip. In contrast, a control strip of chromium coated steel foil whichhad not been subjected to the electrical discharge treatment in theatmosphere of diethylene triamine, showed various spots whereincorrosion had started in this period of time.

Example 9 A three-mil thick strip of copper foil was drawn through theapparatus as in Example 1 wherein the atmosphere was a mixture providedby bubbling nitrogen through glycidyl methacrylate at approximately 4cubic feet per minute and a second gas produced by passing nitrogenthrough ethyleneimine at the same rate. The treated film was laminatedto a S-mil thick film of tetrafluoroethylene/hexafiuoropropene copolymer(weight ratio 85/15) of the type described by Bro and Sandt U.S.P.2,946,763. The lamination was carried out by pressing the films togetherwith a slight pressure (less than 1 pound per square inch at 275 C.). Avery strong bond was obtained; the copper foil was torn in attempting toseparate the copper foil from the plastic film. A laminate made from acontrol copper foil which had been subjected to the electrical dischargebut in the absence of the gaseous atmosphere showed a bond strength ofabout 1500 grams per inch.

Example 10 A strip of fiat silverplate was drawn through an electricdischarge in a -mil gap between spaced electrodes, in an atmosphere ofnitrogen and tetrafluoroethylene, the electric discharge being producedby a motor generator operating at 20,000 cycles frequency and a voltageof 3000 across the electrodes and with the silverplate strip being incontinuous contact with the ground electrode. The treated silverplateshowed no evidence of tarnishing when exposed for one week to anatmosphere containing sulfide vapors, whereas a control sample of thesame silverplate which had not been treated in the electrical dischargebecame blackened when exposed to the same atmosphere for the same periodof time.

It will be evident from the foregoing description and examples that theprocess of this invention results in a wide variety of beneficialeffects on shaped structures having metal surfaces including improvedcorona resistance, resistance to corrosion and chemical attack as wellas improved adherability of the metallic surface to other structures.Moreover, the process is well adapted to large scale industrial use inthat cumbersome and expensive equipment for providing such specialconditions as reduced pressure are not required.

What is claimed is:

1. A process for modifying and improving the characteristics of a metalsurface which comprises subjecting the metal surface of a metal-surfacedshaped structure to the action of an electrical discharge atsubstantially atmospheric pressure in a gaseous atmosphere consisting ofan inert carrier gas which will sustain an electrical discharge and ofthe vapor of at least one organic agent having a vapor pressure of atleast 0.25 mm. of mercury at 60 C. and selected from the groupconsisting of polymerizable organic compounds, nonpolymerizable organiccompounds having replaceable hydrogen and perhalohydrocarbons said vaporbeing present in an amount up to 5% by volume said electrical dischargebeing formed between spaced electrodes to which are applied analternating current at a voltage within the range of from about 1000 to100,000 volts and at a frequency of from about 350 to about 500,000cycles per second effective to create an electrical discharge betweensaid electrodes.

2. A process for modifying and improving the characteristics of a metalsurface which comprises continuously passing a continuous web of apellicular structure having a metal surface between spaced electrodesspaced a distance of from 0.005 inch to 0.25 inch, continuously applyingto said electrodes an alternating current at a voltage within the rangeof from about 0 to 100,000 volts and at a frequency in the range of fromabout 350 to about 500,000 cycles per second effective to create anelectrical discharge between said electrodes and onto said metalsurface, and maintaining between said spaced electrodes at atmosphericpressure an atmosphere consisting of an inert carrier gas which willsustain said electrical discharge and the vapor of at least one organicagent having a vapor pressure of at least 0.25 mm. of mercury at 60 C.,and selected from the group consisting of polymerizable organiccompounds, nonpolymerizable organic compounds having replaceablehydrogen, and perhalohydrocarbons, said vapor being present in an amountup to 5% by volume of said atmosphere.

3. The process of claim 2 wherein said carrier gas is nitrogen.

4. The process of claim 2 wherein said organic agent is a polymerizableorganic monomer.

5. The process of claim 2 wherein said organic agent is acrylonitrile.

6. The process of claim 2 wherein said organic agent is glycidylmethacrylate.

7. The process of claim 2 wherein said pellicular structure is a plasticfilm having on at least one surface thereof a continuous coating ofmetal.

8. The process of claim 7 wherein said plastic film is polyethyleneterephthalate film.

9. The process of claim 8 wherein said metal coating in aluminum.

10. The process of claim 2 wherein said pellicular structure is metalfoil.

References Cited UNITED STATES PATENTS l/ 1959 Kritchever 156-272 X4/1960 Goodman 117-201

